This invention relates to an optical recording and reproducing apparatus and, more particularly, to an optical and reversible recording and reproducing aparatus in which a laser beam is converged to a micro-lightbeam having a diameter of about 1 .mu.m by using a lens or the like and this micro-lightbeam is applied onto an optical recording medium to record and reproduce signals with high density and to erase the recorded signals, thereby permitting repetitional recording and reproducing of signals.
Conventionally, as one example of optical recording and reproducing apparatuses, an apparatus has been proposed and used in which signals are recorded and reproduced with high density by applying a laser beam and the like having the above-mentioned micor-beam diameter on an optical recording disc which is rotating. This kind of optical recording and reproducing apparatus is highlighted as an apparatus which can provide a new recording device and media for future society since it may deal with a large amount of information. The characteristics of apparatus of this kind which permit it to handle large amounts of information include that the recording density is high and, therefore, the memory cost per bit can be reduced since the access time is very short and since an optical head does not come into contact with the tracks of the optical recording disc so that the recording and reproducing can be stably performed.
As the above-mentioned method of optical recording and reproducing, the write-once type and the erasable type have been proposed. As a write-once type recording and reproducing method, there has been proposed a method of recording and reproducing signals in which the thin recording film on a recording disc is locally evaporated by the thermal energy of the laser to form small holes, and a method of recording and reproducing signals in which the optical density of the thin recording film is locally changed by the thermal energy of the laser beam, or the like. On the other hand, as an optical recording and reproducing apparatus of the erasable type, there has been proposed a method in which an optical magnetic recording material to record and reproduce signals in cooperation with the thermal effect of the laser and the external magnetic field is used, and a method in which with respect to the thin recording film a recording disc whose optical density is changed as mentioned above, the optical density is reversibly changed only by using the thermal energy of the laser. It is worth notice in view of such fact that the optical density can be reversibly changed, signals can be erasably recorded and reproduced on and from an optical recording medium.
As one proposed method of reversibly varying the density of the thin recording film, a transition between the amorphous state and the crystal state of the thin recording film, or a transition between one amorphous state and another stable amorphous state is repeatedly carried out. Additionally, there is also proposed a method in which a change in size of the crystal particles in an amorphous matrix is utilized.
Prior to description, it is assumed that the optical density change is obtained, utilizing a transition between the amorphous state and the crystal state for simplicity of description.
FIG. 1 shows a simplified model of the condition for transition between the amorphous state and the crystal state.
In FIG. 1, the amorphous state is indicated by a reference character (A), in which the reflection factor of the light of the thin recording film is small, while the transmission factor of the light is large. The crystal state is represented by (C), in which the reflection factor of the thin recording film is large, but the transmission factor is small. With this thin recording film in which the optical density can be reversibly changed, when the temperatuare of the thin recording film in the amorphous state (A) in FIG. 1 is locally raised up to near its melting point and its portion is slowly cooled, the relevant portion becomes the crystal state (C). On the other hand, when the temperature of the thin recording film in the crystal state is locally raised up to near its melting point and its portion is rapidly cooled, the corresponding portion becomes the amorphous state (A).
FIGS. 2a and 2b show examples of a method of realizing the condition to raise the temperature and cool rapidly and the condition to raise the temperature and cool slowly on the thin recording film.
FIG. 2a shows an almost circular micro spot L to be formed by the laser or the like on the recording medium which relatively moves in the direction indicated by an arrow. When the intensity of this light spot L is enlarged for only a short period of time to raise the temperature of a local portion of the thin film, the thermal energy due to this temperature elevation at this local portion will be rapidly diffused to the thin film and the suporting member of the thin film, thereby providing a rapid cooling condition. In contrast, as shown in FIG. 2b, when an elongated light spot M which extends in the moving direction (indicated by an arrow) of a recording medium is produced in the similar manner by the laser and the like on the recording medium and then the intensity of this light spot M is continuously or intermittently enlarged, the relevant portion of the thin recording film is heated over a wide area. Thus, this heated portion will be more slowly cooled than in the case of FIG. 2a. That is to say, the condition for temperature elevation and rapid cooling is obtained by applying the circular micro-beam on the thin recording film which relatively moves and time-modulates its intensity, thereby to produce the pulse-like light. On one hand, the condition for temperature elevation and slow cooling is obtained by applying the light beam which is elongated in the moving direction of the thin recording disc onto this thin recording film of the recording disc which relatively moves.